Andy, I absolutely see what you want to point out. And honestly, you're right about that. I don't want to badmouth everything from the very first. But a bit of rationality will keep you away from disappointment.

I have a bunch of 1625 and started with a SE in triode connection. I was absolutely unsatisfied with the output power and first thought something else was wrong. But no...you only get a bit over 1W. Yepp...a single watt from a 25W triode. The output power tenfolded when connected as pentode. I liked the increased dynamic range due to the power reserve much more than the triode sound.
The extremely good STC datasheet shows nearly all topologies with that tube: www.tubezone.net/pdf/807stc.pdf

So, no triode bashing from my side, just don't be suprised when the triode power of your 1624 won't be sufficient.

As a triode 1624 has Ra 2.5k mu 10 so it's like a beefed-up 6V6 . Good for a watt or so SE with 8k output transformer . Two in parallel would be more interesting and could use a 3.5k-4k output transformer but very inefficient , a single 2A3 would do better . Ultra linear probably a better application for this valve .

316A

The best application (other than as a Class B or Class C RF amp) is PP, pentode. That's what it was designed to do. If it requires any help, then you can always add local NFB and/or gNFB to take off however much "edge" these produce as PP audio finals.

Getting a watt or two from a tube rated for Pd= 25W that'll have to run at Vpp= 600V is absurd. You'd be better off sticking with actual audio triodes (45, 2A3, 6A3) or vertical deflection triodes. If you're not a "sand-o-phobe", then RF power triodes also become a possibility. The horizontal deflection pents wired as pseudotriodes would also be much better.

If you just have to do it, and can live with the results, well, there have been crazier things done.

Miles - there is indeed a bit of an ominous silence about using the 1624 in triode. Mountains of them about and if it really were a directly heated 807 then surely somebody would have published a few schematics. But they haven't. Mind, nobody used the 2E22 much and that was a much better choice in triode.

It's looking more like a driver in triode if it's anything. I was debating whether to go on with trying this or using 2a3/6B4G outputs exactly like you say, and frankly it's looking like the 2a3s unless anybody has some bright ideas. I'm still curious to lash up something and have a listen, but motivation is getting a bit rocky!!